This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the described embodiments. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior art.
In cementing operations carried out in oil and gas wells, a hydraulic cement composition is disposed between the walls of the wellbore and the exterior of a pipe string, such as a casing string, that is positioned within the wellbore. The cement composition is permitted to set in the annulus thereby forming an annular sheath of hardened, substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the wellbore and bonds the pipe to the walls of the wellbore whereby the migration of fluids between zones or formations penetrated by the wellbore is prevented.
One method of cementing involves pumping the cement composition down through the casing and then up through the annulus. In this method, the volume of cement required to fill the annulus must be calculated. Once the calculated volume of cement has been pumped into the casing, a cement plug is placed in the casing. A drilling mud is then pumped behind the cement plug such that the cement is forced into and up the annulus from the far end of the casing string to the surface or other desired depth. When the cement plug reaches a float shoe disposed proximate the far end of the casing, the cement should have filled the entire volume of the annulus. At this point, the cement is allowed to dry in the annulus into the hard, substantially impermeable mass.
This method, however, may not be suitable for all wells, as it requires the cement to be pumped at high pressures, which makes it potentially unsuitable for wells with softer formations or formation prone to fracture. Reverse cementing is an alternative cementing method in which the cement composition is pumped directly into the annulus between the casing string and the wellbore. Using this approach, the pressure required to pump the cement to the far end of the annulus is much lower than that required in conventional cementing operations. After the cement reaches well bottom, the cement will begin to flow up the inside of the casing string unless pumping is stopped. Thus, in reverse cementing operations, it is necessary to identify when the cement begins to enter the far end of the casing such that the cement pumps may be shut off.